Spray foams have found widespread utility in the fields of insulation and structural reinforcement. For example, spray foams are commonly used to insulate or impart structural strength to items such as automobiles, hot tubs, refrigerators, boats, and building structures. In addition, spray foams are used in applications such as cushioning for furniture and bedding, padding for underlying carpets, acoustic materials, textile laminates, and energy absorbing materials. Currently, spray foams, especially those used as insulators or sealants for home walls, are polyurethane spray foams.
Polyurethane spray foams and their methods of manufacture are well known. Although one-part spray foams are known, more typically, polyurethane spray foams are formed from two separate components, commonly referred to as an “A” side and a “B” side, that react when they come into contact with each other. The first component, or the “A” side, contains an isocyanate such as a di- or poly-isocyanate that has a high level of reactive isocyanate groups (—N═C═O or NCO) on the molecule. The second component, or “B” side, contains nucleophilic reagents such as polyols that include two or more hydroxyl groups, silicone-based surfactants, blowing agents, catalysts, and/or other auxiliary agents. The nucleophilic reagents are generally polyols, primary and secondary polyamines, and/or water. Preferably, mixtures of diols and triols are used to achieve the desired foaming properties. The overall polyol hydroxyl number is designed to achieve a 1:1 ratio of first component to second component (A:B).
The two components are typically delivered through separate lines into a spray gun such as an impingement-type spray gun. The first and second components are pumped through small orifices at high pressure to form separate streams of the individual components. The streams of the first and second components intersect and mix with each other within the gun and begin to react. The heat of the reaction causes the temperature of the reactants in the first and second components to increase. This rise in temperature causes the blowing agent located in the second component (the “B” side) to vaporize and form a foam mixture. As the mixture leaves the gun, the mixture contacts a surface, sticks to it, and continues to react until the isocyanate groups have completely reacted. The resulting resistance to heat transfer, or R-value, may be from 3.5 to 8 per inch.
There are several problems associated with conventional isocyanate-based polyurethane spray foams. One major problem associated with conventional polyurethane spray foams is that the first component (the “A” side) contains high levels of free monomeric methylene-diphenyl-di-isocyanate (MDI). When the foam reactants are sprayed, the MDI monomers form droplets that may be inhaled by workers installing the foam if stringent safety precautions are not followed. Even a brief exposure to isocyanate monomers may cause difficulty in breathing, skin irritation, blistering and/or irritation to the nose, throat, and lungs. Extended exposure of these monomers can lead to a sensitization of the airways, which may result in an asthmatic-like reaction and possibly death.
An additional problem with such conventional polyurethane spray foams is that residual polymeric methylene-diphenyl-di-isocyanate (PMDI) that is not used is considered to be a hazardous waste. PMDI typically has an NCO of about 20%. In addition, PMDI can remain in a liquid state in the environment for years. Therefore, specific procedures must be followed to ensure that the PMDI waste product is properly and safely disposed of in a licensed land fill. Such precautions are both costly and time consuming.
Attempts have been made to reduce or eliminate the presence of free isocyanate monomers and/or isocyanate emission by spray foams into the atmosphere. For example, latex foams have been used to reduce or eliminate the presence of isocyanate and/or isocyanate emission by spray foams. Typical plural component latexes are supplied with a latex as the major component in the “A” side and a crosslinking agent as the minor component in the “B” side. The crosslinking agent in the latex spray foams is generally highly reactive. Thus, the crosslinking agent is generally supplied neat (i.e., not in solution). Additionally, the high reactivity of the crosslinking agent may reduce the stability and result in a short shelf life of the foamable material.
Additionally, attempts have been made to utilize pre-polymerization to lower the concentration of diisocyanate monomers. Although these pre-polymers for spray foams contain low concentrations of diisocyanate monomers, they still contain isocyanate groups. Thus, these foams may be better than conventional polyurethane foams from a toxicological point of view, but are still considered hazardous. In addition, such foams do not solve the waste problems discussed above. Nor have such foams achieved toxicological acceptance.
One attempt to eliminate isocyanate from foams is to make a polyurethane without the use of isocyanate. Such a non-isocyanate polyurethane (NIPU) may be made via the reaction of cyclic carbonates and diamines, as is disclosed in U.S. Pat. No. 5,175,231 to Rappoport, et al.
A further attempt to eliminate the presence of isocyanate groups is to block existing isocyanate groups on the pre-polymers with other reactive and non-hazardous functional groups. One such example is depicted in FIG. 1. Such pre-polymers do not crosslink via isocyanate groups and are thus toxicologically acceptable. In addition, mixtures formed with these blocked pre-polymers can be used to produce spray foams, which in the cured state, may have properties similar to conventional isocyanate-containing polyurethane foams. Further, one-component spray foam systems can be formulated using these blocked pre-polymers that cure exclusively through contact with atmospheric moisture.
Step 1 of FIG. 1 illustrates the reaction of two diisocyanate molecules with a polyol to form an oligomer or pre-polymer. This reaction is repeated until the desired pre-polymer length is achieved. In Step 2, the isocyanate terminated oligomer is reacted with siloxy groups to produce urethane oligomers having siloxy-terminated ends and therefore, the pre-polymer contains no isocyanate end groups. The oligomers are crosslinked by a reaction between the siloxy-terminated groups and water (e.g., moisture in the air).
Despite attempts to reduce or eliminate the use of isocyanate in spray foams and/or reduce isocyanate emission into the air, there remains a need in the art for a spray foam that is non-toxic, environmentally friendly, and stable over time.